The present invention relates generally to the field of electrophotographic imaging techniques, and more particularly, is directed to the use of a foraminated device which is capable of having stored thereon a charge pattern corresponding to the light and dark areas of a graphic original. The device with the charge pattern thereon can then be used to selectively transmit and otherwise block the passage of charged particles directed towards its surface. The charged particles which are permitted to pass through the foraminated device are then collected on a dielectric material and then developed into a visible image. For the purpose of the description of the invention which follows, whenever the term "charged particles" is used, it shall mean both gas ion and toner particles. The term "blocked," whenever used herein, shall define the condition of a charged particle prevented from passing through the apertures in the device due to its being attracted to the surface of the conductor or otherwise repelled from the aperture and returned into the stream of particles directed against the device.
The described foraminated devices are capable of selectively passing or blocking charged particles so that they modulate the flow of a stream of particles projected against its surface. Such devices will be identified as modulators.
Electrophotographic reproduction techniques for making reproductions of graphic originals using photoconductive media are well known. Such processes call for applying a blanket electrostatic charge to a photoconductive layer, and then exposing it to a pattern of light and shadow created by directing electromagnetic radiation onto such graphic original and then projecting the resulting pattern by means of an optical system onto the light-sensitve photoconductive layer. In the light struck areas of the layer, the charges are conducted to ground, leaving behind the charge pattern corresponding to the dark or shadow areas of the graphic original. The images are rendered visible by the application of an electroscopic powder which is then fixed directly on the photoconductive layer or can be transferred and then fixed on a suitable receiving medium such as plain paper. This technique of electrophotographic reproduction has now been adapted for use with foraminated structures or modulators. The modulators of this invention are aperture devices capable of accepting an electrostatic charge and responding to a pattern of light and shadow to have recorded thereon a charge distribution system, which can then be utilized in ordinary room light without affecting the continued existence of the pattern on the surface thereof. In other words, the charge distribution system is, more or less, permanently contained on the modulator.
One system of using such modulators and the apparatus for carrying out such processes is fully described in U.S. Pat. No. 3,986,871 granted on Oct. 19, 1976 and SN 423,884 (abandoned) filed on Dec. 12, 1973, in the names of John D. Blades and Jerome E. Jackson.
To more fully apreciate and understand the modulator structures of this invention, it is necessary to discuss the imaging processes employed in the prior art. The modulators capable of retaining a charge distribution system (CDS) on its surface for long periods of time comprise a photoconductive layer, such as the well-known organic photoconductive material sandwiched between an insulating layer and a metal mesh. Such a three-layered structure has the unique properties of being able to record on the insulating surface a CDS which will endure in ordinary room light, and can be stored for long periods of time.
Such modulator construction permits the creation of a CDS on the insulating layer by first applying a blanket electrostatic charge of one polarity to the surface of the insulating layer and then applying a charge from an AC corona emission electrode, simultaneously projecting thereon a pattern of light and shadow. Alternatively, the charged surface is exposed to a pattern of light and the AC charge shadow and sequentially applied from a corona electrode. This first method results in zero potential in the light-struck areas of the modulator and distributes the charges in the dark areas so that they are at an equal potential level. When using the alternative procedure, the dark areas are at zero potential and distribute the charges in the light-struck areas.
The modulator is given a final flood illumination causing those charges in the photoconductive layer, which correspond to the dark areas of the graphic original, to be conducted to ground leaving a residual charge on the surface of the insulating layer and a corresponding charge bound at the interface between the insulating layer and the photoconductive layer. Such CDS results in electric fields across the apertures, corresponding to the pattern of light and shadow generated by illuminating the graphic original, which are confined to the apertures. These are called fringing fields. These fields do not extend, to any appreciable extent, outside the thickness of the modulator. The electrical fields are the result of the dipole charge created across the insulating layer. When a charged particle or ion comes within the vicinity of an aperture, it encounters the fringing field. It will either be blocked or propelled through the aperture depending on the strength and direction of the field and the polarity of the charge particle.
It is characteristic of gas ions that are produced in air by a suitable high-voltage corona wire or a radioactive source to experience motion which is governed by the electric fields in the vicinity of the ion and by the number of collisions with the air molecules attributable to random thermal motion. The forces operating against the gas ions involved, in both of these situations, are electrical in character. The motion of the gas ions can be controlled by external fields produced by electrodes juxtaposed the modulator. The use of such electrodes, in combination with the modulator, is significant in creating an imaging system whereby charged particles can be projected against the modulator and collected on a suitable medium which becomes the reproduction of the graphic subject matter.
The electrical fields which are produced by such additional electrodes are set up so as to direct a uniform stream of charged particles through the modulator onto a collecting medium or collecting surface. Where the charged particles are toner particles, an image results directly on the collecting medium in conformance to the graphic subject matter and where the particles are gas ions, the collected charges can be developed by a suitable developing means. The electrical fields must be of sufficient strength to overcome the random thermal motion of the ions in the vicinity of the aperture and must be in a direction perpendicular to the ion collecting surface.
To achieve the aforedescribed control with respect to the charged particles, the modulator is placed close to and in a plane parallel to the plane of the charge particle collecting medium. Such a collecting medium is under the influence of a collecting electrode. Hence, to use a modulator with a CDS in an imaging system, the modulator is positioned between an emission electrode establishing a field between the modulator and the electrode, with the latter directing gas ions towards the metal layer of the modulator and a collection electrode positioned on the opposite side of the modulator, facing the insulating surface, which similarly produces a field whose direction is perpendicular to the collecting surface. Associated with the collection electrode is a collecting medium such as a sheet or web of dielectric material in intimate contact with the collecting electrode on which is received the projected charged particles or gas ions which are blocked or transmitted through the modulator.
In the light struck areas, charges are induced in the conductive layer and proceed through the photoconductive layer which is in a conductive state and permits the charge carriers to move through to the interface at the insulating layer.
In the dark areas of the photoconductor, the charges remain trapped. However, the oppositely poled DC corona renders the charges on the insulating surface the same polarity as at the interface.
The modulators described in the aforementioned U.S. patent and abandoned application and successfully be utilized with the processes described therein and provide all of the advantages and improvements over prior art modulator structures and systems. However, the techniques and systems known heretofore for utilizing such three-layered modulators have not been without disadvantages. The deficiencies from which such known procedures suffer relate to the degree of modulation afforded by the system to the charged particles or gas ions presented to the surface and secondly, the period of time necessary to create the charge distribution system (CDS) on the modulator.
The inventions described in U.S. Pat. No. 3,986,871 granted on Oct. 19, 1976 and Ser. No. 423,884 (abandoned) provide for a modulator having a substantially long memory, that is, it is capable of sustaining the CDS on its surface for extended periods of time, even in room light.
The CDS sets up fringing electrical fields in the modulator openings. Such a modulator, in which the photoconductor is sandwiched between the insulating layer and the metal screen, is capable of either blocking the passage of charged particles directed against its surface or permitting the particles to pass through certain areas. The passage of the charged particles through certain openings of the modulators occurs as a function of the natural movement (thermal motion) of the particle within the modulator assisted by the external field forces set by the corona and collecting electrodes. It is also possible to provide a CDS which produces propelling fields or fringing fields in portions of the modulator which exert the necessary force on particles coming within the apertures of the modulator, projecting them through the apertures. In the remaining portions of the modulator which have not been applied a fringing field across the aperture, the charged particles tend to be attracted to the conductive layer, but a certain number of particles move through the aperture by virtue of the external electrodes and their normal movement within the aperture.
Hence, it will be appreciated that the imaging properties of such a modulator can be treated with a CDS so that it either blocks the passage of ions or provides the necessary fields for propelling them through the apertures; but in either case the uncharged portions permit some of the ions to pass through. Understandably, the situation where the ions will form the image, the process is very slow, without a propelling force, in terms of the quantity of charges that can be collected through the modulator as a function of time. In the other case, where there is a field created across the modulator, it speeds up the process, but some of the ions pass through the uncharged portions so that ions go through where ions are not intended to go through.